1. Field of the Invention
The present invention relates to a membrane electrode assembly applied to a solid polymer fuel cell. More particularly, a membrane electrode assembly applied to a solid polymer fuel cell which is suitable for use in a transportable compact power supply, a vehicle-mounted power source, a cogeneration system and the like, and a solid polymer fuel cell using the assembly.
2. Description of Related Art
A solid polymer fuel cell is a fuel cell in which a solid polymer electrolyte membrane is used as an electrolyte. The basic unit thereof, a unitary cell, is constituted of a pair of electrodes bonded respectively to both surfaces of a solid polymer electrolyte membrane (hereinafter, referred to as an xe2x80x9cmembrane electrode assemblyxe2x80x9d.) Each of the electrodes has a two-layer structure consisting of a diffusion layer and a catalyst layer, and the latter is provided on a surface in contact with the solid polymer electrolyte membrane.
The diffusion layer is a layer for supplying a reactant gas to the catalyst layer and exchanging electrons, and it is made from materials having porosity and electron conductivity. The catalyst layer is a layer for causing a catalyst contained therein to initiate electrode reaction. Yielding the electrode reaction requires a three-phase interface where three phases of an electrolyte, a catalyst, and a reactant gas coexist. Therefore, the catalyst layer is typically constituted of a catalyst or a catalyst supported by a catalyst carrier and a porous layer including an electrolyte having the same composition as the solid polymer electrolyte membrane.
By the way, non-crosslinked perfluoro-based electrolytes, typified by Nafion (a registered trademark for products manufactured by E.I. du Pont de Nemours and Company), and a variety of hydrocarbon-based electrolytes have been known as solid polymer electrolytes for use in a solid polymer fuel cell. Any of these, however, needs water for appearance of ion conductivity. Therefore, if the operation of the fuel cell enters a dry condition, so-called dry-up occurs, meaning that water content of a solid polymer electrolyte membrane decreases, and so does electrical conductivity of the membranes, which may become a cause of lowered output of the fuel cell.
On the other hand, if the operation of the fuel cell enters a wet condition, excessive water builds up inside electrodes. In addition, when protons are conducted within a solid polymer electrolyte membrane from one electrode (anode) to the other electrode (cathode), the water also transfers along with the protons to the side of the cathode (this will hereinafter be referred to as xe2x80x9cwater electroosmosisxe2x80x9d.) Besides, in the cathode, water is produced via electrode reaction. If the water is left standing, so-called flooding occurs, meaning that the three-phase interface in the catalyst layer is clogged with the water, which may become a cause of lowered output of the fuel cell.
Accordingly, it is necessary to maintain a solid polymer electrolyte membrane in an appropriate wet state so as to ensure high output with stability from a solid polymer fuel cell. Conventional types of a solid polymer fuel cell have employed a method by which reactant gases supplied to electrodes are humidified with an aid of auxiliary machinery such as a steam generator or a mist atomizer while the amount of humidification is controlled so as to adjust water content of the solid polymer electrolyte membrane (this will hereinafter be referred to as xe2x80x9cwater controlxe2x80x9d.) Besides, there has also been a known method whereby water is injected directly into a reactant gas passages formed within a separator.
However, to make a solid polymer fuel cell more compact and lightweight, it is desired to improve water control property of a membrane electrode assembly so as to lower a degree to which the water control depends on the auxiliary machinery. In order to achieve this, it is considered effective to convert a solid polymer electrolyte membrane into a thin film of high strength. This is because enhancing the strength of a solid polymer electrolyte membrane permits the membrane to be a thin film, which facilitates maintaining the whole of the membrane in a uniform wet state.
As a method of converting a solid polymer electrolyte membrane into a thin film, there have been known several methods including a method whereby an electrolyte is made to contain another crosslinkable polymer for reinforcement (e.g. see Japanese Patent Unexamined Publication Nos. 06(1994)-76838 and 10(1998)-340732), a method whereby a coating of fluorine-based monomer is applied to a reinforcing material constituted of porous fibers and then the monomer is polymerized to introduce ion-exchange groups thereto (e.g. see Japanese Patent Examined Publication No.04(1992)-58822), and a method whereby an electrolyte membrane is bonded to a parfluorocarbon polymer woven fabric by thermal compression to be converted into a multilayer film (e.g. see Japanese Patent Unexamined Publication No. 06(1994)-231780.)
In addition, there has been a known method whereby ion clusters in a perfluoro sulfonic acid membrane are inter-knitted through an inorganic glasslike network in silicon oxide phase (siloxane polymer), silicon oxide+titanium oxide phase, zirconium oxide phase, or the like so as to make a hybrid membrane by sol-gel reactions initiated by immersion of the perfluoro sulfonic acid membrane in an alcohol solution containing alkoxide such as tetraethoxysilane, a mixture of tetrabutyltitanate and tetraethoxysilane, tetrabutylzirconate or the like (e.g. see Journal of Applied Polymer Science, vol.55, p.181 (1995).) Besides, there has also been a known method whereby finely granulated silica and/or fibrous silica fiber is added in order to increase water content and ion conductivity of a solid polymer electrolyte membrane and those of catalyst layers (e.g. see Japanese Patent Unexamined Publication No. 06(1994)-111827.)
Using any of the above-described methods to make a solid polymer electrolyte membrane thinner in thickness and higher in strength, the whole of the membrane can easily be maintained in a comparatively uniform wet state. Flooding or dry-up in a fuel cell, however, arises in dependence on not only the water control property of a solid polymer electrolyte membrane but also that of an electrode.
Accordingly, any of those conventional methods can suppress the flooding and the dry-up resulting from a solid polymer electrolyte membrane to a certain extent, but it is difficult to suppress flooding and dry-up resulting from an electrode. In addition, as an example of giving attention to improving the water control property of an electrode in order to relieve a load on auxiliary machinery at the time of water control, Japanese Patent Unexamined Publication No. 06(1994)-111827 mentions a technique to improve electrical conductivity by adding granulated or fibrous silica. And yet, this technique is not effective enough to greatly improve electrical conductivity since it does not allow of introduction of silica into the inside of conductive paths which are still finer, meaning that this technique does not contribute to the water control property of an electrode.
Such a membrane electrode assembly is commonly produced by application of pressure bonding by hot-pressing to the surfaces of diffusion layers coated with a paste containing a catalyst or a catalyst supported by a catalyst carrier and an electrolyte in the form of a solution. However, conventional types of membrane electrode assemblies present such a problem that bonding failure may occur or that the conductive paths are apt to be discontinuous, because those assemblies are made by merely pressing and bonding electrolytes in the form of solutions contained in catalyst layers to electrolytes in the form of membranes.
On the other hand, in order to solve the problem that output of a fuel cell is lowered by flooding which is caused by excessive water building up particularly inside a cathode with the operating condition of the fuel cell being in a wet condition, it is common practice to employ a method by which the structure of an electrode is optimized. For example, for the purpose of enlarging the three-phase interface, there has been known a method whereby an electrode is made to have the two-layer structure consisting of a diffusion layer and a catalyst layer, the latter of which is provided on the surface in contact with a solid polymer electrolyte membrane. As already described, the diffusion layer is a layer for supplying a reactant gas to the catalyst layer and conducting exchange of electrons, and it is commonly composed of materials having porosity and electron conductivity. The catalyst layer, in turn, is a layer being a reaction site of electrode reaction, and it is commonly composed of a catalyst or a catalyst supported by a catalyst carrier and an electrolyte which has the same composition as the solid polymer electrolyte membrane (an intra-catalyst-layer electrolyte.)
In addition, for example, Japanese Patent Unexamined Publication No. 05(1993)-36418 discloses a porous electrode to which fluororesin is used to adhere catalyst-supporting carbon whose surface is covered with a solid polymer electrolyte for the purpose of enlarging the reaction site for the electrode.
Furthermore, Japanese Patent Unexamined Publication No. 09(1997)-320611 discloses an electrode for use in a solid polymer fuel cell which is obtained by covering a solid polymer electrolyte membrane with a liquid mixture of carbon black supporting a platinum catalyst, fluorine-based ion-exchange resin, and a solvent-soluble fluorine-based polymer having no ion-exchange group, for the purpose of imparting water repellency to the porous electrode uniformly. Besides, this publication also discloses an electrode for use in a solid polymer fuel cell which is obtained by kneading and stretching a mixture of carbon black supporting a platinum catalyst and polytetrafluoroethylene to produce a porous film, and then by impregnating the film with fluorine-based ion-exchange resin and a solvent-soluble fluorine polymer which has no ion-exchange groups.
Humidification of electrolytes with an aid of auxiliary machinery requires various components including a water tank for storing water used in humidification, a humidifier, a condenser for reclaiming water discharged from a fuel cell. This presents a problem that the entire system of the fuel cell should be upsized. Besides, the humidification of electrolytes with the aid of auxiliary machinery needs extra power for the auxiliary machinery, which may become a cause of reduced energy conversion efficiency of the fuel cell.
On the other hand, in a solid polymer fuel cell, as described above, water is produced on the cathode side by electrode reaction. Utilization of the product-water directly for humidifying electrolytes can lessen or eliminate the need for humidifying the electrolytes with auxiliary machinery, promising miniaturization, weight reduction, and high efficiency of the entire system of fuel cell.
Conventional types of electrodes for use in solid polymer fuel cells, however, are not suitable in structure for utilizing the product-water effectively, because the structure is designed to facilitate discharge of water building up inside the electrode, for example by applying water repellency treatment to the surfaces of fine pores in the electrodes.
For example, in an electrode for use in a solid polymer fuel cell disclosed by Japanese Patent Unexamined Publication No. 09(1997)-320611, water repellency is intended to be imparted to an electrode by coating at least a part of surfaces of fine pores in the electrode with a solvent-soluble, fluorine-based polymer which has no ion-exchange group. Even by using the disclosed method, however, it is difficult to cover all ion-exchange groups exposed on the surface of a gas-phase side with a thin and uniform coating of the solvent-soluble fluorine-based polymer. Consequently, most of the product-water is discharged from the inside of the fine pores rather than being effectively utilized for humidifying electrolyte, so that humidification with an aid of auxiliary machinery is essential for stable operation.
The present invention has been made in view of the above circumstances and has an object to overcome the above problems and to provide a membrane electrode assembly in which water control of a solid polymer electrolyte can be performed easily to lighten a load on auxiliary machinery at the time of the water control.
Another object of the present invention is to provide a membrane electrode assembly in which the bonding property of electrodes and a solid polymer electrolyte membrane are enhanced so as to reduce occurrence of bonding failure and that of discontinuousness in conductive paths.
Further, another object of the present invention is to provide a solid polymer fuel cell capable of producing high output with stability even under a no-humidification condition by lessening or eliminating the need for humidifying electrolyte with an aid of auxiliary machinery.
Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, a membrane electrode assembly has a pair of electrodes respectively bonded to both surfaces of a solid polymer electrolyte membrane having a first conductive path, the electrodes each having a catalyst layer. The catalyst layer of at least one of the electrodes has a second conductive path and contains a first metalloxane polymer in an intra-catalyst-layer electrolyte including an electrode catalyst. Desirably, this metalloxane polymer is generated within the second conductive path of the electrolyte, and the metalloxane polymer is added in a state of metalloxane monomer and thereafter polymerized by polycondensation reaction. In this case, the solid polymer electrolyte membrane preferably includes a second metalloxane polymer.
In the membrane electrode assembly according to the present invention, a network structure of the first metalloxane polymer is introduced into the inside of the second conductive path of the catalyst layer. Therefore, high output can be obtained stably regardless of whether the operation of the fuel cell is in a wet condition or a dry condition. This may be ascribable to the following mechanism: an interaction between the catalyst layer and water is increased by the first metalloxane polymer having been introduced into the catalyst layer so that mass transfer is accelerated within the catalyst layer.
In addition, still higher output can be obtained in the case where the second metalloxane polymer is introduced into the solid polymer electrolyte membrane. This may be ascribable to the following mechanism: because of the network structure of the second metalloxane polymer having been introduced into the solid polymer electrolyte membrane, it is possible to make the solid polymer electrolyte thinner in thickness and higher in strength, and to suppress water electroosmosis. As a result, water content of the solid polymer electrolyte membrane is maintained appropriately.
According to the membrane electrode assembly of the present invention, the first metalloxane polymer is contained in the intra-catalyst-layer electrolyte in the catalyst layer of at least one of the electrodes, which are bonded to both surfaces of the solid polymer electrolyte. The presence of the first metalloxane polymer produces such an effect to accelerate mass transfer within the catalyst layer, and thereby improving water control property of the electrode.
In addition, in the case where the solid polymer electrolyte membrane further contains the second metalloxane polymer, distribution of water within the solid polymer electrolyte membrane can be made uniform easily, which produces such an effect that water control property of the solid polymer electrolyte membrane improves.
Further, the first metalloxane polymer generated within the catalyst layer enhances the bonding property of the electrode and the solid polymer electrolyte membrane, producing such an effect that it is made possible to obtain a membrane electrode assembly having an excellent output performance.
Furthermore, in this type of membrane electrode assembly obtained by bonding electrodes to both surfaces of a solid polymer electrolyte membrane, it is preferred that a gas-phase surface of the intra-catalyst-layer electrolyte containing the first metalloxane polymer be covered with a water-repellent layer having gas-permeability.
Covering the gas-phase surface of the intra-catalyst-layer electrolyte with the water-repellent layer having gas-permeability suppresses discharge of water from the catalyst layer to the diffusion layer without inhibiting a reactant gas from diffusing to the catalyst layer. Consequently, a part of the water produced by electrode reaction remains within the catalyst layer. The residual water within the catalyst layer is then put back into the solid polymer electrolyte membrane by diffusion, and reused to humidify the solid polymer electrolyte membrane. Therefore, even under low-humidification or no-humidification conditions, water content of the solid polymer electrolyte membrane can be maintained at a level sufficient enough for stable operation, thereby ensuring high output with stability.
Thus, in the solid polymer fuel cell comprising the membrane electrode assembly in which a pair of the electrodes are bonded respectively to both surfaces of the solid polymer electrolyte membrane, the membrane is maintained in an appropriate wet state even under the condition where no humidification is performed with an aid of auxiliary machinery, which produces the effect that high output is ensured, if at least one of the electrodes comprises a catalyst layer containing a catalyst or a catalyst supported by a carrier and an intra-catalyst-layer electrolyte, and if the gas-phase surface of the intra-catalyst-layer electrolyte is covered with the water-repellent layer having gas-permeability.